High strength steel sheet and method for manufacturing the same

ABSTRACT

An object is to provide a high strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability and a method for manufacturing the steel sheet.A high strength steel sheet which is excellent in terms of formability, which is manufactured under optimized manufacturing conditions, and which has a predetermined chemical composition and a steel microstructure including, in terms of area fraction, 35% or more and 80% or less of ferrite, 5% or more and 35% or less of as-quenched martensite, 0.1% or more and less than 3.0% of tempered martensite, and 8% or more of retained austenite, in which the average grain size of the ferrite is 6 μm or less, in which the average grain size of the retained austenite is 3 μm or less, in which a value calculated by dividing the average Mn content in the retained austenite by the average Mn content in the ferrite is 1.5 or more, in which a value calculated by dividing the sum of the area fraction of as-quenched martensite having a circle-equivalent grain size of 3 μm or more and the area fraction of retained austenite having a circle-equivalent grain size of 3 μm or more by the sum of the area fraction of all the as-quenched martensite and the area fraction of all the retained austenite is less than 0.4, and in which a value calculated by dividing the area fraction of retained austenite grains adjacent to three or more ferrite grains having different crystal orientations by the area fraction of all the retained austenite is less than 0.6.

TECHNICAL FIELD

The present invention relates to a high strength steel sheet which canpreferably be used as a material for members in industrial fields suchas automotive and electrical industrial fields and which is excellent interms of formability, and relates to a method for manufacturing thesteel sheet, and in particular, is intended to obtain a high strengthsteel sheet which has a TS (tensile strength) of 980 MPa or more andwhich is excellent not only in terms of ductility but also in terms ofhole expansion formability and bendability.

BACKGROUND ART

Nowadays, from the viewpoint of global environment conservation, anincrease in the fuel efficiency of an automobile is an important issueto be addressed. Therefore, there is an active trend toward increasingthe strength of a material for an automobile body to decrease thethickness of the material, thereby decreasing the weight of theautomobile body. However, since an increase in the strength of a steelsheet causes a deterioration in formability, there is a demand for thedevelopment of a material having not only high strength but also highformability.

As a steel sheet which is excellent not only in terms of high strengthbut also in terms of high ductility, a high strength steel sheetutilizing the strain-induced transformation of retained austenite isproposed. Since such a steel sheet has a steel microstructure includingretained austenite, it is possible to easily form the steel sheet due toretained austenite when forming is performed, and it is possible toachieve high strength due to retained austenite transforming intomartensite after forming has been performed.

For example, Patent Literature 1 proposes a high strength steel sheethaving significantly high ductility which contains Mn in an amount of0.2 weight % to 2.5 weight %, which has a tensile strength of 1000 MPaor more and an EL (total elongation) of 30% or more, and which utilizesthe strain-induced transformation of retained austenite. Such a steelsheet is manufactured by forming austenite in a steel sheet containingC, Si, and Mn as basic composition and by thereafter performing aso-called austempering treatment, in which the steel sheet is subjectedto quenching and isothermal holding in a temperature range for bainitetransformation. In this austempering treatment, retained austenite isformed due to an increase in the C concentration in austenite, and alarge amount of C of more than 0.3% is necessary to form a large amountof retained austenite. However, in the case where the C concentration insteel is high, there is a deterioration in spot weldability, and inparticular, there is a marked deterioration in spot weldability in thecase where the C concentration is more than 0.3%, which results indifficulty in using the steel sheet practically for an automobile. Inaddition, since the main object of Patent Literature 1 is to improve theductility of a high strength steel sheet, no consideration is given tohole expansion formability or bendability.

In Patent Literature 2, by performing a heat treatment in a temperaturerange for forming a ferrite-austenite dual phase on steel containing Mnin an amount of 4 weight % to 6 weight %, a high level ofstrength-ductility balance is achieved. However, in the case of PatentLiterature 2, since no consideration is given to an improvement inductility due to an increase in the Mn concentration in untransformedaustenite, there is room for an improvement in workability.

In addition, in Patent Literature 3, by performing a heat treatment in atemperature range for forming a ferrite-austenite dual phase on steelcontaining Mn in an amount of 3.0 mass % to 7.0 mass % to increase theMn concentration in untransformed austenite, stable retained austeniteis formed, thereby improving total elongation. However, since the heattreatment time is short, it is inferred that there is an insufficientincrease in the Mn concentration due to the low diffusion rate of Mn.

Moreover, in Patent Literature 4, by performing heat treatment, for along time, in a temperature range for forming a ferrite-austenite dualphase on a hot rolled steel sheet containing Mn in an amount of 0.50mass % to 12.00 mass % to increase the Mn concentration in untransformedaustenite, retained austenite having a large aspect ratio is formed,which results in an improvement in uniform elongation and hole expansionformability. However, in Patent Literature 4, consideration is givenonly to an improvement in the ductility and hole expansion formabilityof a high strength steel sheet, and no consideration is given to animprovement in hole expansion formability or bendability through thecontrol of dispersion conditions in a second phase including retainedaustenite and martensite.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 61-157625

PTL 2: Japanese Unexamined Patent Application Publication No. 1-259120

PTL 3: Japanese Unexamined Patent Application Publication No.2003-138345

PTL 4: Japanese Patent No. 6123966

SUMMARY OF INVENTION Technical Problem

The present invention has been completed in view of the situationdescribed above, and an object of the present invention is to provide ahigh strength steel sheet which has a TS (tensile strength) of 980 MPaor more and which is excellent in terms of formability, and inparticular, not only in terms of ductility but also in terms of holeexpansion formability and bendability and a method for manufacturing thesteel sheet.

Solution to Problem

To solve the problems described above, the present inventors diligentlyconducted investigations from the viewpoints of the chemical compositionof a steel sheet and a method for manufacturing the steel sheet and, asa result, found the following.

The chemical composition of a steel material is controlled to contain Mnin an amount of 2.50 mass % or more and 8.00 mass % or less with thecontents of other alloy elements such as Ti being appropriatelycontrolled as needed, the steel material is subjected to hot rolling,and the hot rolled steel sheet is held in a temperature range equal toor lower than the Ac₁ transformation temperature for more than 1800 s asneeded, is thereafter subjected to pickling as needed, and is thensubjected to cold rolling. Consequently, the steel sheet is held in atemperature range equal to or higher than the Ac₃ transformationtemperature for 20 s to 1800 s, the steel sheet is cooled to atemperature of 50° C. or higher and 350° C. or lower, the cooled steelsheet is held at the cooling stop temperature for 2 s to 600 s, thesteel sheet is cooled, and a pickling treatment is then performed asneeded. Subsequently, the steel sheet is held in a temperature rangeequal to or higher than the Ac₁ transformation temperature and equal toor lower than (Ac₁ transformation temperature+150° C.) for 20 s to 1800s, and the steel sheet is cooled, subjected to a pickling treatment asneeded, then preferably held in a temperature range equal to or higherthan the Ac₁ transformation temperature and equal to or lower than (Ac₁transformation temperature+150° C.) for 20 s to 1800 s, and then cooled.As a result, it was found that it is possible to manufacture a highstrength steel sheet excellent in terms of formability having a steelmicrostructure including, in terms of area fraction, 35% or more and 80%or less of ferrite, 5% or more and 35% or less of as as-quenchedmartensite, 0.1% or more and less than 3.0% of tempered martensite, and8% or more of retained austenite, in which the average grain size of theferrite is 6 μm or less, in which the average grain size of the retainedaustenite is 3 μm or less, in which a value calculated by dividing theaverage Mn content (mass %) in the retained austenite by the average Mncontent (mass %) in the ferrite is 1.5 or more, in which a valuecalculated by dividing the sum of the area fraction of as-quenchedmartensite having a circle-equivalent grain size of 3 μm or more and thearea fraction of retained austenite having a circle-equivalent grainsize of 3 μm or more by the sum of the area fraction of all theas-quenched martensite and the area fraction of all the retainedaustenite is less than 0.4, and in which a value calculated by dividingthe area fraction of retained austenite grains adjacent to three or moreferrite grains having different crystal orientations by the areafraction of all the retained austenite is less than 0.6.

The present invention has been completed on the basis of the knowledgedescribed above, and the subject matter of the present invention is asfollows.

[1] A high strength steel sheet having a chemical compositioncontaining, by mass %, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn:2.50% to 8.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005%to 0.0100%, Al: 0.001% to 2.000%, and a balance being Fe and incidentalimpurities and, a steel microstructure including, in terms of areafraction, 35% or more and 80% or less of ferrite, 5% or more and 35% orless of as-quenched martensite, 0.1% or more and less than 3.0% oftempered martensite, and 8% or more of retained austenite, in which anaverage grain size of the ferrite is 6 μm or less, in which an averagegrain size of the retained austenite is 3 μm or less, in which a valuecalculated by dividing an average Mn content (mass %) in the retainedaustenite by an average Mn content (mass %) in the ferrite is 1.5 ormore, in which a value calculated by dividing a sum of an area fractionof as-quenched martensite having a circle-equivalent grain size of 3 μmor more and an area fraction of retained austenite having acircle-equivalent grain size of 3 μm or more by a sum of an areafraction of all the as-quenched martensite and an area fraction of allthe retained austenite is less than 0.4, and in which a value calculatedby dividing an area fraction of retained austenite grains adjacent tothree or more ferrite grains having different crystal orientations bythe area fraction of all the retained austenite is less than 0.6.

[2] The high strength steel sheet according to item [1], in which thechemical composition further contains, by mass %, at least one selectedfrom Ti: 0.005% to 0.200%, Nb: 0.005% to 0.200%, V: 0.005% to 0.500%, W:0.005% to 0.500%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%, Cr:0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002%to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.100%, Ca: 0.0005% to0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM:0.0005% to 0.0050%.

[3] The high strength steel sheet according to item [1] or [2], in whichthe steel sheet has a galvanizing layer on a surface of the steel sheet.

[4] The high strength steel sheet according to item [3], in which thegalvanizing layer is a galvannealing layer.

[5] A method for manufacturing a high strength steel sheet, the methodincluding heating a steel slab having the chemical composition accordingto item [1] or [2], performing hot rolling on the heated slab with afinish rolling delivery temperature of 750° C. or higher and 1000° C. orlower, coiling the hot rolled steel sheet at a temperature of 300° C. orhigher and 750° C. or lower, performing cold rolling on the hot rolledsteel sheet, subsequently holding the cold rolled steel sheet in atemperature range equal to or higher than an Ac₃ transformationtemperature for 20 s to 1800 s, cooling the steel sheet to a coolingstop temperature of 50° C. or higher and 350° C. or lower, holding thecooled steel sheet at the cooling stop temperature for 2 s to 600 s,cooling the steel sheet, subsequently holding the cooled steel sheet ina temperature range equal to or higher than an Ac₁ transformationtemperature and equal to or lower than (Ac₁ transformationtemperature+150° C.) for 20 s to 1800 s, and cooling the steel sheet.

[6] A method for manufacturing a high strength steel sheet, the methodincluding heating a steel slab having the chemical composition accordingto item [1] or [2], performing hot rolling on the heated slab with afinish rolling delivery temperature of 750° C. or higher and 1000° C. orlower, coiling the hot rolled steel sheet at a temperature of 300° C. orhigher and 750° C. or lower, performing cold rolling on the hot rolledsteel sheet, subsequently holding the cold rolled steel sheet in atemperature range equal to or higher than an Ac₃ transformationtemperature for 20 s to 1800 s, cooling the steel sheet to a coolingstop temperature of 50° C. or higher and 350° C. or lower, holding thecooled steel sheet at the cooling stop temperature for 2 s to 600 s,cooling the steel sheet, subsequently holding the cooled steel sheet ina temperature range equal to or higher than an Ac₁ transformationtemperature and equal to or lower than (Ac₁ transformationtemperature+150° C.) for 20 s to 1800 s, cooling the steel sheet, againholding the cooled steel sheet in the temperature range equal to orhigher than the Ac₁ transformation temperature and equal to or lowerthan (Ac₁ transformation temperature+150° C.) for 20 s to 1800 s, andcooling the steel sheet.

[7] The method for manufacturing a high strength steel sheet accordingto item [5] or [6], the method further including, after coiling has beenperformed, holding the steel sheet in a temperature range equal to orlower than the Ac₁ transformation temperature for more than 1800 s.

[8] The method for manufacturing a high strength steel sheet accordingto any one of items [5] to [7], the method further including performinga galvanizing treatment.

[9] The method for manufacturing a high strength steel sheet accordingto item [8], the method further including, after the galvanizingtreatment has been performed, performing an alloying treatment at atemperature of 450° C. to 600° C.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain a highstrength steel sheet which has a TS (tensile strength) of 980 MPa ormore and which is excellent in terms of formability, and in particular,not only in terms of ductility but also in terms of hole expansionformability and bendability. By using a high strength steel sheetmanufactured by using the manufacturing method according to the presentinvention for, for example, automobile structural members, it ispossible to improve fuel efficiency due to a decrease in the weight ofan automobile body, which has a significant utility value in theindustry.

DESCRIPTION OF EMBODIMENTS

Hereafter, the present invention will be specifically described. Here,“%” used when describing the contents of the composition of a chemicalcomposition denotes “mass %”, unless otherwise noted.

(1) The reasons why the chemical composition of steel is specified asdescribed above in the present invention will be described.

C: 0.030% or More and 0.250% or Less

C is an element which is necessary to increase strength by formingmartensite. In addition, C is an element which is effective forimproving the ductility of steel by improving the stability of retainedaustenite. In the case where the C content is less than 0.030%, since itis difficult to achieve the desired area fraction of martensite, it isnot possible to achieve the desired strength. In addition, since it isdifficult to achieve a sufficient area fraction of retained austenite,it is not possible to achieve good ductility. On the other hand, in thecase where the C content is excessive and is more than 0.250%, sincethere is an excessive increase in the area fraction of hard martensite,there is an increase in the number of micro voids at the crystal grainboundaries of martensite and crack propagation progresses when a holeexpanding test is performed, which results in a deterioration in holeexpansion formability. In addition, since there is a deterioration inthe mechanical properties of a weld zone due to a significant increasein the hardness of the weld zone and a heat-affected zone (HAZ), thereis a deterioration in spot weldability, arc weldability, and so forth.From such viewpoints, the C content is set to be 0.030% or more and0.250% or less. It is preferable that the C content be 0.080% or more.It is preferable that the C content be 0.200% or less. Hereafter, theterm “hard martensite” denotes as-quenched martensite (martensiteas-quenched state).

Si: 0.01% or More and 3.00% or Less

Since Si improves the work hardenability of ferrite, Si is effective forachieving good ductility. Since there is a decrease in the effect of Siincluded in the case where the Si content is less than 0.01%, the lowerlimit of the Si content is set to be 0.01%. However, in the case wherethe Si content is excessive and is more than 3.00%, embrittlement occursin steel, and there is a deterioration in surface quality due to, forexample, the occurrence of red scale. Moreover, there is a deteriorationin phosphatability and quality of coating. Therefore, the Si content isset to be 0.01% or more and 3.00% or less. It is preferable that the Sicontent be 0.20% or more. It is preferable that the Si content be 2.00%or less or more preferably less than 0.70%.

Mn: 2.50% or More and 8.00% or Less

Mn is a significantly important composition element in the presentinvention. Mn is an element which stabilizes retained austenite, whichis effective for achieving good ductility, and which increases thestrength of steel through solid solution strengthening. Such effects arerealized in the case where the Mn content in steel is 2.50% or more.However, in the case where the Mn content is excessive and is more than8.00%, there is a deterioration in phosphatability and quality ofcoating. From such viewpoints, the Mn content is set to be 2.50% or moreand 3.00% or less. It is preferable that the Mn content be 3.10% or moreor more preferably 3.20% or more. It is preferable that the Mn contentbe 6.00% or less or more preferably 4.20% or less.

P: 0.001% or more and 0.100% or less

P is an element which has the function of solid solution strengtheningand which may be included in accordance with desired strength. Inaddition, P is an element which is effective for forming a multi-phasestructure by promoting ferrite transformation. To realize such effects,it is necessary that the P content be 0.001% or more. On the other hand,in the case where the P content is more than 0.100%, there is adeterioration in weldability, and there is a deterioration ingalvanizing layer quality due to a decrease in alloying rate when agalvanizing layer is subjected to an alloying treatment. Therefore, theP content is set to be 0.001% or more and 0.100% or less. It ispreferable that the P content be 0.005% or more. It is preferable thatthe P content be 0.050% or less.

S: 0.0001% or More and 0.0200% or Less

S causes embrittlement to occur in steel when hot work is performed as aresult of being segregated at grain boundaries and causes adeterioration in local deformability as a result of existing in the formof sulfides. Therefore, it is necessary that the S content be 0.0200% orless, preferably 0.0100% or less, or more preferably 0.0050% or less.However, due to constraints regarding manufacturing techniques, it isnecessary that the S content be 0.0001% or more. Therefore, the Scontent is set to be 0.0001% or more and 0.0200% or less. It ispreferable that the S content be 0.0001% or more. It is preferable thatthe S content be 0.0100% or less or more preferably 0.0050% or less.

N: 0.0005% or More and 0.0100% or Less

N is an element which causes a deterioration in the aging resistance ofsteel. In particular, in the case where the N content is more than0.0100%, there is a marked deterioration in aging resistance. Althoughit is preferable that the N content be as small as possible, it isnecessary that the N content be 0.0005% or more due to constraintsregarding manufacturing techniques. Therefore, the N content is set tobe 0.0005% or more and 0.0100% or less. It is preferable that the Ncontent be 0.0010% or more. It is preferable that the N content be0.0070% or less.

Al: 0.001% or More and 2.000% or Less

Al is an element which is effective for decreasing theannealing-temperature dependency of mechanical properties, that is, forstabilizing material properties by expanding a temperature range forforming a ferrite-austenite dual phase. Since there is a decrease in theeffect of Al included in the case where the Al content is less than0.001%, the lower limit of the Al content is set to be 0.001%. Inaddition, since Al is an element which is effective for increasing thecleanliness of steel by functioning as a deoxidizing agent, it ispreferable that Al be included in a deoxidizing process. However, in thecase where the Al content is more than 2.000%, since there is anincreased risk of a crack occurring in a steel slab when continuouscasting is performed, there is a deterioration in manufacturability.From such viewpoints, the Al content is set to be 0.001% or more and2.000% or less. It is preferable that the Al content be 0.200% or more.It is preferable that the Al content be 1.200% or less.

In addition, by mass %, at least one selected from Ti: 0.005% or moreand 0.200% or less, Nb: 0.005%, or more and 0.200% or less, V: 0.005% ormore and 0.500% or less, W: 0.005% or more and 0.500% or less, B:0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000% orless, Cr: 0.005% or more and 1.000% or less, Mo: 0.005% or more and1.000% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% ormore and 0.200% or less, Sb: 0.002% or more and 0.200% or less, Ta:0.001% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% orless, Mg: 0.0005% or more and 0.0050% or less, Zr: 0.0005% or more and0.0050% or less, and REM (abbreviation for rare earth metal): 0.0005% ormore and 0.0050% or less may optionally be included in addition to thecomposition described above.

Compositions other than those described above are Fe and incidentalimpurities. Here, in the case where one of the compositions describedabove is contained in an amount less than the lower limit of the contentthereof, such a composition is regarded as being contained as anincidental impurity.

Ti: 0.005% or More and 0.200% or Less

Since Ti is effective for increasing the strength of steel throughprecipitation strengthening, Ti decreases the difference in hardnessbetween ferrite and a hard second phase (martensite or retainedaustenite) by increasing the strength of ferrite, thereby making itpossible to achieve good hole expansion formability. It is possible torealize such an effect in the case where the Ti content is 0.005% ormore. However, in the case where the Ti content is more than 0.200%,since there is an excessive increase in the area fraction of hardmartensite, there is an increase in the number of micro voids at thecrystal grain boundaries of martensite and crack propagation progresseswhen a hole expanding test is performed, which may result in adeterioration in hole expansion formability (for blanking). Therefore,in the case where Ti is included, the Ti content is set to be 0.005% ormore and 0.200% or less. It is preferable that the Ti content be 0.010%or more. It is preferable that the Ti content be 0.100% or less.

Nb: 0.005% or More and 0.200% or Less, V: 0.005% or More and 0.500% orLess, and W: 0.005% or More and 0.500% or Less

Nb, V, and W are effective for increasing the strength of steel throughprecipitation strengthening, and it is possible to realize such aneffect in the case where the content of each of these elements is 0.005%or more. In addition, as in the case of the effect of Ti beingcontained, these elements decrease the difference in hardness betweenferrite and a hard second phase (martensite or retained austenite) byincreasing the strength of ferrite, thereby making it possible toachieve good hole expansion formability. It is possible to realize suchan effect in the case where the content of each of these elements is0.005%, or more. However, in the case where the Nb content is more than0.200% or the V content or the W content is more than 0.500%, sincethere is an excessive increase in the area fraction of hard martensite,there is an increase in the number of micro voids at the crystal grainboundaries of martensite and crack propagation progresses when a holeexpanding test is performed, which may result in a deterioration in holeexpansion formability. Therefore, in the case where Nb is included, theNb content is set to be 0.005% or more and 0.200% or less. It ispreferable that the Nb content be 0.010% or more. It is preferable thatthe Nb content be 0.100% or less. In the case where V or W is contained,the content of V or W is set to be 0.005% or more and 0.500% or less. Itis preferable that the content of V or W be 0.010% or more. It ispreferable that the content of V or W be 0.300% or less.

B: 0.0003% or More and 0.0050% or Less

Since B has the function of inhibiting the formation and growth offerrite from austenite grain boundaries, B decreases the difference inhardness between ferrite and a hard second phase (martensite or retainedaustenite) by increasing the strength of ferrite, thereby making itpossible to achieve good hole expansion formability. It is possible torealize such an effect in the case where the B content is 0.0003% ormore. However, in the case where the B content is more than 0.0050%,there may be a deterioration in formability. Therefore, in the casewhere B is contained, the B content is set to be 0.0003% or more and0.0050% or less. It is preferable that the B content be 0.0005% or more.It is preferable that the B content be 0.0030% or less.

Ni: 0.005% or More and 1.000% or Less

Ni is an element which stabilizes retained austenite, which is effectivefor achieving good ductility, and which increases the strength of steelthrough solid solution strengthening. It is possible to realize such aneffect in the case where the Ni content is 0.005% or more. On the otherhand, in the case where the Ni content is more than 1.000%, since thereis an excessive increase in the area fraction of hard martensite, thereis an increase in the number of micro voids at the crystal grainboundaries of martensite and also crack propagation progresses when ahole expanding test is performed, which may result in a deterioration inhole expansion formability. Therefore, in the case where Ni iscontained, the Ni content is set to be 0.005% or more and 1.000% orless. It is preferable that the Ni content be 0.010% or more. It ispreferable that the Ni content be 0.500% or less.

Cr: 0.005% or More and 1.000% or Less and Mo: 0.005% or More and 1.000%or Less

Since Cr and Mo have the function of improving a strength-ductilitybalance, these elements may be included as needed. It is possible torealize such an effect in the case where the Cr content is 0.005% ormore or the Mo content is 0.005% or more. However, in the case where theCr content is excessive and is more than 1.000% or the Mo content isexcessive and is more than 1.000%, since there is an excessive increasein the area fraction of hard martensite, there is an increase in thenumber of micro voids at the crystal grain boundaries of martensite andalso crack propagation progresses when a hole expanding test isperformed, which may result in a deterioration in hole expansionformability. Therefore, in the case where these elements are included,the Cr content is set to be 0.005% or more and 1.000% or less, and theMo content is set to be 0.005% or more and 1.000% or less. It ispreferable that the Cr content be 0.010% or more. It is preferable thatthe Cr content be 0.500%, or less. It is preferable that the Mo contentbe 0.010% or more. It is preferable that the Mo content be 0.500% orless.

Cu: 0.005% or More and 1.000% or Less

Cu is an element which is effective for increasing the strength ofsteel. It is possible to realize such an effect in the case where the Cucontent is 0.005% or more. On the other hand, in the case where the Cucontent is more than 1.000%, since there is an excessive increase in thearea fraction of hard martensite, there is an increase in the number ofmicro voids at the crystal grain boundaries of martensite and also crackpropagation progresses when a hole expanding test is performed, whichmay result in a deterioration in hole expansion formability. Therefore,in the case where Cu is contained, the Cu content is set to be 0.005% ormore and 1.000% or less. It is preferable that the Cu content be 0.010%or more. It is preferable that the Cu content be 0.500% or less.

Sn: 0.002% or More and 0.200% or Less and Sb: 0.002% or More and 0.200%or Less

Sn and Sb are included as needed from the viewpoint of inhibitingdecarburization in a region within about several tens of μm of thesurface of the steel sheet due to the nitridation and oxidation of asteel sheet surface. In the case where the Sn content is 0.002% or moreor the Sb content is 0.002% or more, it is possible to inhibit suchnitridation and oxidation, thereby inhibiting a decrease in the areafraction of martensite on the steel sheet surface, which is effectivefor achieving satisfactory strength and the stability of materialproperties. On the other hand, in the case where the Sn content or theSb content is excessive and is more than 0.200%, there is adeterioration in toughness. Therefore, in the case where Sn and Sb areincluded, the content of each of these elements is set to be 0.002% ormore and 0.200% or less. It is preferable that the content of each of Snand Sb be 0.004% or more. It is preferable that the content of each ofthese elements be 0.050% or less. Here, the term “martensite” denotesas-quenched martensite.

Ta: 0.001% or More and 0.100% or Less

Ta, like Ti and Nb, contributes to increasing strength by forming alloycarbides and alloy carbonitrides. In addition, it is considered that,since Ta is partially dissolved in Nb carbides and Nb carbonitrides toform complex precipitates such as (Nb, Ta) (C, N), Ta markedly inhibitscoarsening of precipitates, thereby stabilizing the contribution toincreasing strength through precipitation strengthening. Therefore, itis preferable that Ta be contained. Here, it is possible to realize theabove-described effect of stabilizing precipitates in the case where theTa content is 0.001% or more. On the other hand, in the case where theTa content is excessively high, the effect of stabilizing precipitatesbecomes saturated, and there is an increase in alloy cost. Therefore, inthe case where Ta is included, the Ta content is set to be 0.001% ormore and 0.100% or less. It is preferable that the Ta content be 0.005%or more. It is preferable that the Ta content be 0.050% or less.

Ca: 0.0005% or More and 0.0050% or Less, Mg: 0.0005% or More and0.0050%, or Less, Zr: 0.0005% or More and 0.0050% or Less, and REM:0.0005% or More and 0.0050% or Less

Ca, Mg, Zr and REM are elements which are effective for further reducingthe negative effect of sulfides on hole expansion formability throughthe spheroidizing of sulfides. To realize such an effect, it ispreferable the content of each of these elements be 0.0005% or more.However, in the case where the content of each of these elements isexcessive and is more than 0.0050%, there is an increase in the amountof, for example, inclusions, which results in surface and internaldefects. Therefore, in the case where Ca, Mg, Zr, and REM are included,the content of each of these elements is set to be 0.0005% or more and0.0050% or less. It is preferable that the content of each of Ca, Mg,Zr, and REM be 0.0010% or more. It is preferable that the content ofeach of Ca, Mg, Zr, and REM be 0.0040% or less.

(2) Hereafter, the Microstructure Will be Described.

Area Ratio of Ferrite: 35% or More and 80% or Less

To achieve satisfactory ductility, it is necessary that the areafraction of ferrite be 35% or more. In addition, to achieve a tensilestrength of 980 MPa or more, it is necessary that the area fraction ofsoft ferrite be 80% or less. Here, the meaning of “ferrite” includespolygonal ferrite, granular ferrite, and acicular ferrite, which arecomparatively soft and excellent in terms of ductility. It is preferablethat the area fraction of ferrite be 40% or more. It is preferable thatthe area fraction of ferrite be 75% or less.

Area Fraction of as-Quenched Martensite: 5% or More and 35% or Less

To achieve a TS of 980 MPa or more, it is necessary that the areafraction of as-quenched martensite be 5% or more. In addition, toachieve good ductility, it is necessary that the area fraction ofas-quenched martensite be 35% or less. It is preferable that the areafraction of as-quenched martensite be 5% or more. It is preferable thatthe area fraction of as-quenched martensite be 30% or less.

Area Fraction of Tempered Martensite: 0.1% or More and Less than 3.0%

To achieve good hole expansion formability, it is necessary that thearea fraction of tempered martensite be 0.1% or more. In addition, toachieve a TS of 980 MPa or more, it is necessary that the area fractionof tempered martensite be less than 3.0%. It is preferable that the areafraction of tempered martensite be 0.1% or more. It is preferable thatthe area fraction of tempered martensite be 2.0% or less.

Incidentally, it is possible to derive the area fractions of ferrite,as-quenched martensite, and tempered martensite by polishing a crosssection (L-cross section) in the thickness direction parallel to therolling direction of a steel sheet, by etching the polished crosssection with a 3 vol. % nital solution, by observing 10 fields of viewat a position located at ¼ of the thickness (position located at adistance of ¼ of the thickness from the steel sheet surface, where thedistance is measured in the thickness direction) with a SEM (scanningelectron microscope) at a magnification of 2000 times to obtainmicrostructure images, by analyzing the obtained microstructure imagesby using Image-Pro produced by Media Cybernetics, Inc. to calculate thearea fraction of each of the microstructures (ferrite, martensite, andtempered martensite) in each of the 10 fields of view, and bycalculating the average area fraction of each of the microstructuresfrom the area fractions of each of the microstructures in the 10 fieldsof view. In addition, in the microstructure image described above,ferrite is identified as a gray microstructure (base microstructure),martensite is identified as a white microstructure, and temperedmartensite is identified as a white martensite microstructure containinggray substructures.

Area Fraction of Retained Austenite: 8% or More

To achieve satisfactory ductility, it is necessary that the areafraction of retained austenite be 8% or more or preferably 12% or more.

Here, the area fraction of retained austenite is defined as a volumefraction which is derived by polishing a steel sheet to a plane located0.1 mm from a position located at ¼ of the thickness, by furtherperforming chemical polishing on the polished surface to remove athickness of 0.1 mm, by determining the integral intensity ratio of thediffraction peak of each of the (200)-plane, (220)-plane, and(311)-plane of fcc (face centered cubic) iron and the (200)-plane,(211)-plane, and (220)-plane of bcc (body centered cubic) iron of theexposed surface by using an X-ray diffractometer with CoKα-ray, and bycalculating the average value of the obtained 9 integral intensityratios to derive a volume fraction.

Average Grain Size of Ferrite: 6 μm or Less

A grain refinement of ferrite contributes to an improvement in TS.Therefore, to achieve the desired TS, it is necessary that the averagegrain size of ferrite be 6 μm or less or preferably 5 μm or less.

Average Grain Size of Retained Austenite: 3 μm or Less

A grain refinement of retained austenite contributes to an improvementin ductility and hole expansion formability. Therefore, to achieve goodductility and hole expansion formability, it is necessary that theaverage grain size of retained austenite be 3 μm or less or preferably2.5 μm or less.

Value Calculated by Dividing Average Mn Content (Mass %) in RetainedAustenite by Average Mn Content (Mass %) in Ferrite: 1.5 or More

The requirement that the value calculated by dividing the average Mncontent (mass %) in retained austenite by the average Mn content (mass%) in ferrite be 1.5 or more is a significantly important feature in thepresent invention. To achieve good ductility, it is necessary that thearea fraction of stable retained austenite in which Mn is concentratedbe high. It is preferable that such a value be 2.0 or more.

It is possible to derive the Mn content in retained austenite byquantifying Mn distribution in each of the phases in the cross sectionin the rolling direction at a position located at ¼ of the thickness byusing an FE-EPMA (field emission-electron probe micro analyzer) and bycalculating the average Mn content in randomly selected 30 retainedaustenite grains and the average Mn content in randomly selected 30ferrite grains in the measurement field of view.

Value calculated by dividing sum of area fraction of as-quenchedmartensite having a circle-equivalent grain size of 3 μm or more andarea fraction of retained austenite having a circle-equivalent grainsize of 3 μm or more by sum of area fraction of all as-quenchedmartensite and area fraction of all retained austenite: less than 0.4

The requirement that a value calculated by dividing the sum of the areafraction of as-quenched martensite having a circle-equivalent grain sizeof 3 μm or more and the area fraction of retained austenite having acircle-equivalent grain size of 3 μm or more by the sum of the areafraction of all the as-quenched martensite and the area fraction of allthe retained austenite be less than 0.4 is an important feature in thepresent invention. A decrease in the grain sizes of as-quenchedmartensite and retained austenite contributes to an improvement in holeexpansion formability. To achieve good hole expansion formability, whileforming sufficient amounts of as-quenched martensite and retainedaustenite for achieving high strength and high ductility, it isnecessary to increase the area fractions of fine as-quenched martensiteand retained austenite. It is preferable that such a value be less than0.3% or more preferably less than 0.2.

Here, the average grain sizes of ferrite, martensite, and retainedaustenite are derived by determining the area of each of ferrite grains,martensite grains, and retained austenite grains by using Image-Prodescribed above, by calculating circle-equivalent grain sizes, and bycalculating the average circle-equivalent grain size of each of thephases. Martensite and retained austenite are distinguished by usingPhase Map of EBSD (electron backscattered diffraction).

Value calculated by dividing area fraction of retained austenite grainsadjacent to three or more ferrite grains having different crystalorientations by area fraction of all retained austenite: less than 0.6The requirement that a value calculated by dividing the area fraction ofretained austenite grains adjacent to three or more ferrite grainshaving different crystal orientations, that is, retained austenitegrains existing at the triple points of ferrite grain boundaries, by thearea fraction of all the retained austenite be less than 0.6 is animportant feature in the present invention. A decrease in the number offerrite grains which are adjacent to retained austenite grains and whichhave different crystal orientations causes the relax of stressconcentration at the time of hole expansion deformation and bendingdeformation, thereby contributing to an improvement in hole expansionformability and bendability. Therefore, to achieve good hole expansionformability, it is necessary that the area fraction of retainedaustenite grains adjacent to three or more ferrite grains havingdifferent crystal orientations be low. It is preferable that such avalue be less than 0.5.

Here, the crystal orientation of ferrite is determined by using InversePole Figure Map of EBSD (electron backscattered diffraction). Inaddition, the expression “ferrite grains having different crystalorientations” denotes a case where ferrite grains have a misorientationof 1 degree or more in terms of Euler angle obtained by performing EBSDanalysis. In addition, retained austenite grains adjacent to three ormore ferrite grains having different crystal orientations are identifiedby using an IPF map obtained by performing EBSD analysis.

In addition, it is preferable that a value calculated by dividing thearea fraction of massive austenite by the sum of the area fraction oflath-structured austenite and the area fraction of massive austenite beless than 0.6. In the case where the area fraction of massive austeniteis excessively large, there may be a deterioration in the hole expansionformability of steel. Therefore, to achieve better hole expansionformability, it is preferable that a value calculated by dividing thearea fraction of massive austenite by the sum of the area fraction oflath-structured austenite and the area fraction of massive austenite beless than 0.6. It is more preferable that a value calculated by dividingthe area fraction of massive austenite by the sum of the area fractionof lath-structured austenite and the area fraction of massive austenitebe less than 0.4. The term “massive austenite” denotes an austenitegrain having an aspect ratio between major and minor axes of less than2.0, and the term “lath-structured austenite” denotes an austenite grainhaving an aspect ratio between major and minor axes of 2.0 or more.Here, the aspect ratio of retained austenite is calculated by drawing anellipse circumscribed around the retained austenite grain by usingPhotoshop elements 13 and by dividing the major axis of the ellipse bythe minor axis of the ellipse.

Even in the case where, in addition to ferrite, as-quenched martensite,tempered martensite, and retained austenite, bainite, pearlite, andcarbides such as cementite are included in a total amount of 10% or lessin terms of area fraction in the microstructure according to the presentinvention, there is no decrease in the effects of the present invention.

(3) Hereafter, the Manufacturing Conditions Will be Described.

Steel Slab Heating Temperature

Although there is no particular limitation on the heating temperature ofa steel slab, it is preferable that the heating temperature be 1100° C.or higher and 1300° C. or lower. Since precipitates existing in thesteel slab heating stage exist in the form of precipitates having alarge grain size in a finally obtained steel sheet, such precipitates donot contribute to strength, and it is possible to redissolve Ti- andNb-based precipitates which has been precipitated when casting isperformed. Moreover, from the viewpoint of removing blowholes,segregated materials, and so forth in the surface layer of the steelslab to decrease the number of cracks and unevenness on the steel sheetsurface and thereby achieving a higher level of smooth steel sheetsurface, it is preferable that the steel slab heating temperature be1100° C. or higher. On the other hand, from the viewpoint of inhibitingan increase in the amount of scale loss due to an increase in the amountof oxidation, it is preferable that the steel slab heating temperaturebe 1300° C. or lower. It is more preferable that the steel slab heatingtemperature be 1150° C. or higher. It is more preferable that the steelslab heating temperature be 1250° C. or lower.

Although it is preferable that a steel slab be manufactured by using acontinuous casting method from the viewpoint of inhibiting macrosegregation, for example, an ingot casting method or a thin-slab castingmethod may be used. In addition, not only a conventional method, inwhich, after having manufactured a steel slab, the slab is first cooledto room temperature and then reheated, but also an energy-saving processsuch as a hot direct rolling process, in which a slab in the hot stateis charged into a heating furnace without being cooled to roomtemperature and then subjected to hot rolling, or a hot charge rollingor hot direct rolling, in which a slab is rolled immediately after heatretention has been performed for a short time may be used withoutcausing any problem. In addition, a steel slab is made into a sheet barby performing rough rolling under ordinary conditions, and, in the casewhere a heating temperature is comparatively low, it is preferable thatthe sheet bar be heated by using, for example, a bar heater beforefinish rolling is performed from the viewpoint of inhibiting problemsfrom occurring when hot rolling is performed.

Finish rolling delivery temperature of hot rolling: 750° C. or higherand 1000° C. or lower

The steel slab which has been subjected to heating is subjected to hotrolling through a rough rolling process and a finish rolling process sothat a hot rolled steel sheet is obtained. At this time, in the casewhere the delivery temperature is higher than 1000° C., since there is arapid increase in the amount of oxides (scale) generated, the interfacebetween the base steel and the oxides is damaged, which results in atendency for the surface quality to be deteriorated after pickling andcold rolling have been performed. In addition, in the case where, forexample, hot rolling scale is partially left unremoved after picklinghas been performed, there is a negative effect on ductility and holeexpansion formability. Moreover, since there is an excessive increase ingrain size, there may be a case where a surface defect occurs in apressed product when forming is performed. On the other hand, in thecase where the delivery temperature is lower than 750° C., there is anincrease in load on the rolling process due to an increase in rollingload, and there is an increase in rolling reduction ratio under thecondition in which austenite is not recrystallized. As a result, sinceit is not possible to achieve the desired grain size, there is a markedin-plane anisotropy in a final product due to the growth of an abnormaltexture, which results in a deterioration not only in the uniformity(stability) in material properties but also in ductility. Therefore, itis necessary that the finish rolling delivery temperature of hot rollingbe 750° C. or higher and 1000° C. or lower. It is preferable that thefinish rolling delivery temperature of hot rolling be 750° C. or higher.It is preferable that the finish rolling delivery temperature of hotrolling be 950° C. or lower.

Coiling Temperature after Hot Rolling: 300° C. or Higher and 750° C. orLower

In the case where the coiling temperature after hot rolling has beenperformed is higher than 750° C., since there is an increase in thegrain size of ferrite in the hot rolled steel sheet microstructure, itis difficult to achieve the desired strength of a final annealed steelsheet. On the other hand, in the case where the coiling temperatureafter hot rolling has been performed is lower than 300° C., since thereis an increase in the strength of the hot rolled steel sheet, there isan increase in rolling load in a cold rolling process and there is adeterioration in a steel sheet shape, resulting in a deterioration inproductivity. Therefore, it is necessary that the coiling temperatureafter hot rolling has been performed be 300° C. or higher and 750° C. orlower. It is preferable that the coiling temperature after hot rollinghas been performed be 400° C. or higher. It is preferable that thecoiling temperature after hot rolling has been performed be 650° C. orlower.

Incidentally, in a hot rolling process, finish rolling may becontinuously performed by connecting steel sheets which have beensubjected to rough rolling. In addition, the steel sheet which has beensubjected to rough rolling may be coiled. In addition, to decrease arolling load when hot rolling is performed, lubrication rolling may beperformed in part or all of the finish rolling. It is also preferablethat lubrication rolling be performed from the viewpoint of uniformshape and uniform material properties of a steel sheet. Here, in thecase where lubrication rolling is performed, it is preferable that thefriction coefficient be 0.10 or more and 0.25 or less.

Thus obtained hot rolled steel sheet is optionally subjected topickling. It is preferable that pickling be performed, because thismakes it possible to remove oxides from the steel sheet surface, whichresults in an improvement in phosphatability and quality of coating. Inthe case where a hot rolled steel sheet is heated and held, to removeoxides on the steel sheet surface, pickling may be performed once afterheating and holding followed by cooling have been performed, or thepickling process may be divided into multiple times. In the case wherethe pickling process is divided into multiple times, it is preferablethat pickling be performed after heating and holding followed by coolinghave been performed, because this makes it possible to more effectivelyremove oxides on the steel sheet surface. In the case where heating andholding is performed plural times, pickling may be performed each timeafter heating and holding followed by cooling have been performed.

Holding in a temperature range equal to or lower than the Ac₁transformation temperature for more than 1800 s

It is preferable that a hot rolled steel sheet be held in a temperaturerange equal to or lower than the Ac₁ transformation temperature for morethan 1800 s, because this softens the steel sheet which is to besubjected to a subsequent cold rolling process.

In the case where the steel sheet is held in a temperature range equalto or lower than the Ac₃ transformation temperature, since Mn isconcentrated in austenite, hard martensite and retained austenite areformed after cooling has been performed. As a result, it is possible torealize a preferable condition in which a value calculated by dividingthe sum of the area fraction of as-quenched martensite having acircle-equivalent grain size of 3 μm or more and the area fraction ofretained austenite having a circle-equivalent grain size of 3 μm or moreby the sum of the area fraction of all the as-quenched martensite andthe area fraction of all the retained austenite is less than 0.30. Inaddition, in the case where holding is performed for less than 1.800 s,since it is not possible to remove strain due to hot rolling, there maybe a case where the steel sheet is not softened.

Incidentally, as a heat treatment method, any one of a continuousannealing method and a batch annealing method may be used. In addition,when cooling is performed to room temperature after the heat treatmenthas been performed, there is no particular limitation on the method orcooling rate used for cooling, any one of furnace cooling and aircooling in batch annealing, gas jet cooling, mist cooling, and watercooling in continuous annealing, and so forth may be performed. Inaddition, when a pickling treatment is performed, a common method may beused.

Cold Rolling

The obtained steel sheet is subjected to cold rolling. Although there isno particular limitation on the cold rolling reduction ratio, it ispreferable that the cold rolling reduction ratio be 15% to 80%. Byperforming cold rolling with a cold rolling reduction ratio in such arange, since it is possible to achieve a sufficiently recrystallizeddesired microstructure, there is an improvement in ductility.

Holding in temperature range equal to or higher than Ac₃ transformationtemperature for 20 s to 1800 s

In the case where the steel sheet is held in a temperature range lowerthan the Ac₃ transformation temperature or for less than 20 s, sincerecrystallization does not sufficiently progress, it is not possible toachieve the desired microstructure, which results in a deterioration inλ (blanking) and bendability. In addition, the surface concentration ofMn does not sufficiently progress for achieving quality of coating insubsequent processes. On the other hand, in the case where holding isperformed for more than 1800 s, the effect due to surface concentrationof Mn becomes saturated.

Cooling to cooling stop temperature of 50° C. or higher and 350° C. orlower and holding at cooling stop temperature for 2 s to 600 s

The technological thought of the present invention is characterized inthat, by forming thin film-structured austenite (nucleation site ofaustenite which is less likely to come into contact with ferrite) in amicrostructure before annealing is performed, such film-structuredaustenite is made into lath-structured austenite (austenite which isless likely to come into contact with ferrite) in the subsequentannealing process, and Mn is concentrated in such lath-structuredaustenite. By cooling the steel sheet to a cooling stop temperature of50° C. or higher and 350° C. or lower and by holding the steel sheet atthe cooling stop temperature, film-structured austenite, which is madeinto lath-structured austenite in the subsequent annealing process, isformed. In the case where holding is performed at a temperature of lowerthan 50° C., since martensite transformation is completed, there is nofilm-structured austenite left, which results in lath-structuredaustenite not being achieved. In addition, in the case where holding isperformed at a temperature of higher than 350° C., film-structuredaustenite is decomposed, which results in lath-structured austenite notbeing achieved. Therefore, in the case where holding is performed at atemperature of lower than 50° C. or higher than 350° C., since it is notpossible to achieve retained austenite, which is less likely to comeinto contact with ferrite, a large number of retained austenite grainshaving three or more different crystal orientations are formed at grainboundaries and the triple points of grain boundaries in the subsequentannealing process. As a result, since there is an increase in the numberof retained austenite grains which are likely to come into contact withthree or more ferrite grains having different crystal orientations inrelation to the number of retained austenite grains which are lesslikely to come into contact with the above-described three or moreferrite grains having different crystal orientations, it is not possibleto achieve the desired microstructure. In addition, also in the casewhere holding is performed for less than 2 s, since it is not possibleto achieve retained austenite which is less likely to come into contactwith ferrite, it is not possible to achieve the desired microstructure.Moreover, also in the case where holding is performed for more than 600s, since retained austenite which is less likely to come into contactwith ferrite is decomposed, there is an increase in the number ofretained austenite grains which are likely to come into contact withthree or more ferrite grains having different crystal orientations,which results in the desired microstructure not being achieved.

Holding in temperature range equal to or higher than Ac₁ transformationtemperature and equal to or lower than (Ac₁ transformationtemperature+150° C.) for 20 s to 0.1800 s

The requirement that holding be performed in a temperature range equalto or higher than the Ac₁ transformation temperature and equal to orlower than (Ac₁ transformation temperature+150° C.) for 20 s to 1800 sis a significantly important feature in the present invention. In thecase where holding is performed in a temperature range lower than theAc₁ transformation temperature or for less than 20 s, since carbideswhich are formed when heating is performed are left undissolved, it isdifficult to achieve sufficient area fractions of martensite andretained austenite, which results in a deterioration in strength. Inaddition, in the case where holding is performed in a temperature rangehigher than (Ac₁ transformation temperature+150° C.), since the effectof Mn concentration in austenite becomes saturated, it is not possibleto achieve a sufficient area fraction of retained austenite, whichresults in a deterioration in ductility. It is preferable that holdingbe performed in a temperature range equal to or lower than (Ac₁transformation temperature+100° C.). Moreover, in the case where holdingis performed for more than 1800 s, there is an increase in the amount ofmartensite and higher strength, and it is not possible to achieve asufficient area fraction of retained austenite for achievingsatisfactory ductility.

In addition, following cooling to a cooling stop temperature afterhaving performed holding in a temperature range equal to or higher thanthe Ac₁ transformation temperature and equal to or lower than (Ac₁transformation temperature+150° C.) for 20 s to 1300 s, by againperforming holding in a temperature range equal to or higher than theAc₁ transformation temperature and equal to or lower than (Ac₁transformation temperature+150° C.) for 20 s to 1800 s, it is possibleto achieve good material properties. In the case where holding isperformed in a temperature range lower than the Ac₁ transformationtemperature or for less than 20 s, since it is not possible to achievesufficient area fractions of martensite and retained austenite, there isa deterioration in ductility. On the other hand, in the case whereholding is performed in a temperature range higher than the Ac₁transformation temperature or for more than 1800 s, since there is anexcessive increase in the area fraction of martensite, there is adeterioration in ductility. It is preferable that the above-describedcooling stop temperature be 200° C. or lower in the case where picklingis performed or 200° C. to 500° C. in the case where pickling is notperformed. Here, although there is no particular limitation on themethod used for cooling to the above-described cooling stop temperature,air cooling may be performed.

Galvanizing Treatment

In the case where a galvanizing treatment is performed, the steel sheetwhich has been subjected to an annealing treatment as described above isdipped in a galvanizing bath having a temperature of 440° C. or higherand 500° C. or lower to perform a galvanizing treatment, and a coatingweight is then adjusted by using, for example, a gas wiping method.Here, it is preferable that a galvanizing bath containing 0.08% or moreand 0.30% or less of Al be used for a galvanizing treatment.

In the case where an alloying treatment is performed on a galvanizinglayer, an alloying treatment is performed on the galvanizing layer in atemperature range of 450° C. or higher and 600° C. or lower. In the casewhere an alloying treatment is performed at a temperature of higher than600° C., since untransformed austenite transforms into pearlite, it isnot possible to achieve the desired area fraction of retained austenite,which may result in a deterioration in ductility. Therefore, in the casewhere an alloying treatment is performed on a galvanizing layer, it ispreferable that an alloying treatment be performed in a temperaturerange of 450° C. or higher and 600° C. or lower.

Although there is no particular limitation on the other conditionsapplied for the manufacturing method, it is preferable that theabove-described annealing treatment (heating and holding) be performedby using continuous annealing equipment in terms of productivity. Inaddition, it is preferable that a series of treatments including anannealing treatment, a galvanizing treatment, an alloying treatment on agalvanizing layer be performed by using a CGL (continuous galvanizingline), which is a galvanizing treatment line.

Incidentally, skin pass rolling may be performed on a “high strengthsteel sheet” and a “high strength galvanized steel sheet” describedabove for the purpose of correcting a shape, adjusting surfaceroughness, and so forth. It is preferable that the rolling reductionratio of skin pass rolling be 0.1% or more and 2.0% or less. In the casewhere the rolling reduction ratio is lower than 0.1%, there is aninsufficient effect, and there is a difficulty in control. Therefore,the preferable lower limit is set to be 0.11. In addition, in the casewhere the rolling reduction ratio is higher than 2.0%, there is a markeddeterioration in productivity. Therefore, the preferable upper limit isset to be 2.0%. Here, skin pass rolling may be performed online oroffline. In addition, skin pass rolling may be performed once to obtainthe target rolling reduction ratio, or a skin pass rolling may bedivided into several times. In addition, various kinds of coatingtreatments such as oil coating and resin coating may be performed.

EXAMPLES

Steels having chemical compositions given in Table 1 with a balancebeing Fe and incidental impurities were obtained by steelmaking by usinga converter and made into steel slabs by using a continuous castingmethod. The obtained steel slabs were reheated to a temperature of 1250°C., subjected to hot rolling, optionally subjected to a heat treatmentin a temperature range equal to or lower than the Ac₁ transformationtemperature, subjected to cold rolling, subjected to heating and holdingin a temperature range equal to or higher than the Ac₃ transformationtemperature, cooled, and subjected to annealing in a temperature rangeequal to or higher than the Ac₁ transformation temperature and equal toor lower than (Ac₁ transformation temperature+150° C.) to obtain highstrength cold rolled steel sheets (CR) under the conditions given inTables 2 and 3. Moreover, the obtained cold rolled steel sheets weresubjected to a galvanizing treatment to obtain galvanized steel sheets(GI) and galvannealed steel sheets (GA). Here, in the case where anannealing treatment in a temperature range equal to or higher than theAc₁ transformation temperature and equal to or lower than (Ac₁transformation temperature+150° C.) was performed twice, cooling to roomtemperature was performed after the first annealing treatment had beenperformed, and the second annealing treatment was performed thereafter.The galvanizing bath for galvanized steel sheets (GI) contained Al: 0.19mass %, and the galvanizing bath for galvannealed steel sheets (GA)contained Al: 0.14 mass %. The galvanizing baths had a temperature of465° C. The coating weight was 45 g/m2 per side (double-sided coating),and, in the case of GA, the Fe concentration in the coating layer wasadjusted to be 9 mass % or more and 12 mass % or less.

The cross-sectional microstructure, tensile properties, and holeexpansion formability of obtained steel sheets were investigated, andthe results are given in Tables 4, 5, and 6.

Incidentally, the Ac₁ transformation temperature and the Ac₃transformation temperature were calculated by using the followingequations.

Ac ₁ transformation temperature(° C.)=751−16×(% C)+11×(% Si)−28×(%Mn)−5.5×(% Cu)−16×(% Ni)+13×(% Cr)+3.4×(% Mo)

Ac ₃ transformation temperature(° C.)=910−203×√(% C)+45×(% Si)−30×(%Mn)−20×(% Cu)−15×(% Ni)+11×(% Cr)+32×(% Mo)+104×(% V)+400×(% Ti)+200×(%Al)

Here, each of (% C), (% Si), (% Mn), (% Ni), (% Cu), (% Cr), (% Mo), (%V), (% Ti), and (% Al) denotes the content (mass %) of the correspondingelement.

TABLE 1 A_(C) ₁ A_(D) ₃ Trans- Trans- form- form- ation ation St- Tem-Tem- eel per- per- Gr- Chemical Composition (mass %) ature ature ade CSi Mn P S N Al Ti Nb V W B Ni Cr Mo Cu Sn Sb Ta Ca Mg Zr REM (° C.) (°C.) Note A 0.163 0.55 3.56 0.021 0.0022 0.0034 0.031 0.048 — — — — — — —— — — — — — — — 655 771 Example Steel B 0.182 0.82 3.24 0.025 0.00250.0040 0.048 0.042 — — — — — — — — — — — — — — — 666 789 Example Steel C0.175 1.64 3.61 0.020 0.0019 0.0028 0.033 0.032 — — — — — — — — — — — —— — — 667 819 Example Steel D 0.235 0.99 3.32 0.020 0.0029 0.0044 0.041— — — — — — — — — — — — — — — — 665 765 Example Steel E 0.034 0.77 4.110.022 0.0024 0.0041 0.032 — — — — — — — — — — — — — — — — 644 790Example Steel F 0.181 2.89 3.99 0.025 0.0019 0.0021 0.040 0.043 — — — —— — — — — — — — — — — 668 842 Example Steel G 0.199 0.59 3.54 0.0240.0020 0.0025 0.040 0.050 — — — — — — — — — — — — — — — 655 748 ExampleSteel H 0.062 1.01 5.10 0.028 0.0027 0.0026 0.035 — — — — — — — — — — —— — — — — 618 751 Example Steel I 0.185 1.49 3.77 0.023 0.0022 0.00260.034 — — — — — — — — — — — — — — — — 659 783 Example Steel J 0.160 0.203.49 0.031 0.0023 0.0035 0.031 0.046 — — — — — — — — — — — — — — — 653739 Example Steel K 0.127 0.37 5.81 0.027 0.0026 0.0031 0.030 0.052 — —— — — — — — — — — — — — — 590 686 Example Steel L 0.192 0.45 3.19 0.0220.0026 0.0025 0.034 — — — — — — — — — — — — — — — — 664 752 ExampleSteel M 0.151 0.61 4.16 0.022 0.0025 0.0037 0.036 — — — — — — — — — — —— — — — — 639 741 Example Steel N 0.170 0.53 3.20 0.030 0.0020 0.00320.038 0.040 — — — — — — — — — — — — — — — 665 762 Example Steel O 0.2060.33 3.62 0.026 0.0026 0.0042 0.223 0.032 — — — — — — — — — — — — — — —650 781 Example Steel P 0.201 4.08 3.48 0.028 0.0027 0.0035 0.031 — — —— — — — — — — — — — — — — 695 904 Comp- arative Steel Q 0.181 0.63 8.130.026 0.0023 0.0027 0.034 — — — — — — — — — — — — — — — — 527 615 Comp-arative Steel R 0.171 0.60 3.88 0.019 0.0019 0.0040 0.042 0.389 — — — —— — — — — — — — — — — 646 900 Comp- arative Steel S 0.179 0.80 3.490.029 0.0025 0.0040 0.045 0.013 0.043 — — — — — — — — — — — — — — 659770 Example Steel T 0.198 1.14 3.59 0.031 0.0024 0.0027 0.045 — — 0.59 —— — — — — — — — — — — — 660 778 Example Steel U 0.111 1.21 4.07 0.0270.0025 0.0045 0.044 — — — 0.018 — — — — — — — — — — — — 649 783 ExampleSteel V 0.138 0.30 4.07 0.031 0.0022 0.0044 0.040 0.019 — — — 0.0019 — —— — — — — — — — — 638 741 Example Steel W 0.198 0.67 3.94 0.026 0.00230.0040 0.032 0.066 — — — — 0.301 — — — — — — — — — — 640 760 ExampleSteel X 0.094 0.50 3.37 0.020 0.0019 0.0037 0.041 0.062 — — — — — 0.031— — — — — — — — — 661 778 Example Steel Y 0.099 1.40 3.08 0.029 0.00230.0032 0.032 0.024 — — — — — — 0.062 — — — — — — — — 679 834 ExampleSteel Z 0.107 0.52 3.58 0.025 0.0023 0.0027 0.040 — — — — — — — — 0.273— — — — — — — 655 762 Example Steel AA 0.116 0.54 3.17 0.025 0.00210.0033 0.035 0.035 — — — — — — — — 0.005 — — — — — — 666 790 ExampleSteel AB 0.159 0.44 3.20 0.020 0.0025 0.0026 0.0034 0.059 — — — — — — —— — 0.009 — — — — — 664 795 Example Steel AC 0.193 0.68 3.57 0.0160.0026 0.0030 0.033 — — — — — — — — — — — 0.006 — — — — 655 751 ExampleSteel AD 0.201 0.39 3.22 0.031 0.0023 0.0026 0.041 — 0.038 — — — — — — —0.006 — — — — — — 662 748 Example Steel AE 0.212 0.26 3.71 0.023 0.00270.0040 0.032 — 0.032 — — — — — — — — — 0.007 — — — — 647 723 ExampleSteel AF 0.210 0.95 3.97 0.024 0.0025 0.0040 0.040 — — — — — — — — — — —— 0.0031 — — — 647 749 Example Steel AG 0.194 1.25 3.83 0.022 0.00240.0036 0.035 — — — — — — — — — — — — — 0.0020 — — 654 769 Example SteelAH 0.240 0.02 3.11 0.026 0.0023 0.0028 0.040 0.011 — — — — — — — — — — —— — 0.0030 — 666 730 Example Steel AI 0.081 0.04 4.11 0.021 0.00280.0036 0.048 — — — — — — — — — — — — — — — 0.0029 635 740 Example SteelUnderlined portion: out of the range of the present invention. “—”denotes a content of a level of an incidental impurtiy.

TABLE 2 Finish Hot Rolled Steel Sheet Rolling Heat TreatmentCold-rolled-sheet Annealing Treatment Cold-rolled-sheet AnnealingCold-rolled-sheet Annealing Delivery Coiling Heat Cold Heat CoolingCooling Stop Treatment Treatment Alloying Temp- Temp- Treatment HeatRolling Treatment Heat Stop Temperature- Heat Treatment Heat HeatTreatment Heat Temp- Steel erature erature Temperature TreatmentReduction Temperature Treatment Temperature Holding TemperatureTreatment Temperature Treatment erature No. Grade (° C.) (° C.) (° C.)Time(s) Ratio (%) (° C.) Time(s) (° C.) Time(s) (° C.) Time(s) (° C.)Time(s) (° C.) Kind* Note 1 A 890 500 560 18000 61.1 850 150 350 180 690210 CR Example 2 A 880 450 510 36000 52.9 820 180 180 250 680 150 700180 GI Example 3 A 860 480 590 23400 56.3 800 120 150 130 690 180 680150 GI Example 4 A 900 460 550 28800 64.7 825 150 120 180 700 120 720300 550 GA Example 5 A 880 460 520  9000 57.1 850 120 160 130 690 180690 150 530 GA Example 6 A 890 450 530 32400 58.8 800 300 200 360 710360 CR Example 7 A 910 500 540 36000 66.7 830 140 280 500 720 300 700210 GI Example 8 A 870 500 540 10800 66.7 830 140 140 510 685 200 720240 GI Example 9 A 880 550 570 18000 53.3 850 150 140 500 700 210 730160 500 GA Example 10 A 900 530 500  7200 58.8 870 180 300 240 710 180710 140 560 GA Example 11 A 910 520 600 23400 62.5 610 150 200 180 685240 680 120 GI Comparative Example 12 A 920 500 570 14400 58.8 880 120450 140 680 330 690 130 520 GA Comparative Example 13 A 870 540 53018000 57.1 850 200  20 260 730 190 720 120 500 GA Comparative Example 14A 875 480 530  9000 61.1 800 360 220 900 705 210 CR Comparative Example15 A 880 510 46.2 830 250 170 500 685 180 690 140 540 GA Example 16 B890 500 550 21600 54.8 840 360 180 540 690 120 680 150 GI Example 17 C900 520 570 21600 52.9 850 150 250 180 690 210 705 175 550 GA Example 18A 710 540 570 18000 47.1 860 180 140 255 730 250 740 250 530 GAComparative Example 19 A 910 850 590 36000 56.5 870 210 180 260 660 150670 180 510 GA Comparative Example 20 A 880 600 720 21600 58.8 880 180110 250 700 140 700 150 490 GA Comparative Example 21 A 890 590 53014400 52.9 840 140 100 180 580 190 680 230 GI Comparative Example 22 A880 580 52.9 860 130 170 175 810 240 680 150 GI Comparative Example 23 A880 530 520 23400 57.1 830 120 170  40 740 15 CR Comparative Example 24A 880 560 600 36000 51.7 800 360 100 530 700 2400 710 350 GI ComparativeExample 25 A 860 560 600 10800 58.8 805 300 120 370 705 120 550 300 500GA Comparative Example 26 A 870 540 540 18000 68.4 800 360 180 625 710180 860 460 490 GA Comparative Example 27 A 850 480 520  7200 61.1 810150 150 170 680 60 680 10 520 GA Comparative Example 28 A 860 580 55023400 64.7 820 180 110 260 710 150 710 3600 540 GA Comparative Example29 D 900 560 580 14400 58.8 880 210 170 270 680 170 CR Examle 30 E 870550 570 18000 58.8 900 360 160 530 690 180 680 150 GI Example 31 F 905600 580 32400 57.1 810 150 180 255 690 120 CR Example 32 F 890 600 57032400 57.1 800 120 100 130 680 70 700 240 560 GA Example 33 G 875 500525 18000 53.3 820 120 150 150 690 180 680 90 510 GA Example underlinedportion: out of the range of the present invention CR: cold rolled steelsheet (without a coating layer). GI: galvanized steel sheet (without analloying treatment on a galvanizing layer). GA: galvannealed steel sheet

TABLE 3 Finish Hot Rolled Steel Sheet Rolling Heat TreatmentCold-rolled-sheet Annealing Treatment Cold-rolled-sheet AnnealingCold-rolled-sheet Annealing Delivery Coiling Heat Cold Heat CoolingCooling Stop Treatment Treatment Alloying Temp- Temp- Treatment HeatRolling Treatment Heat Stop Temperature- Heat Treatment Heat HeatTreatment Heat Temp- Steel erature erature Temperature TreatmentReduction Temperature Treatment Temperature Holding TemperatureTreatment Temperature Treatment erature No. Grade (° C.) (° C.) (° C.)Time(s) Ratio (%) (° C.) Time(s) (° C.) Time(s) (° C.) Time(s) (° C.)Time(s) (° C.) Kind* Note 34 H 910 560 530 23400 50.0 780 150 150 150680 210 700 360 530 GA Example 35 I 880 500 510 28800 52.9 800 180 130190 710 180 CR Example 36 J 870 560 520 32400 48.6 790 900 140 120 690 80 740 120 GI Example 37 K 910 580 560 23400 46.2 780 180 170 290 630130 640 200 GI Example 38 L 880 610 580 36000 62.5 810 150 130 190 680190 710 250 550 GA Example 39 L 880 600 530 28800 62.5 800 140 130 180690 180 CR Example 40 M 880 520 520 10800 58.8 820  75 200 120 670 170660 300 500 GA Example 41 N 860 480 52.0 825 120 150 140 590 140 590 360520 GA Example 42 O 870 560 600  7200 56.3 830 300 190 400 700  80 CRExample 43 P 890 600 550  7200 62.5 820 150 130 190 680 190 710 250 550GA Comparative Example 44 P 880 600 540 10800 62.5 830 160 150 180 690180 CR Comparative Example 45 Q 850 480 510 10800 64.7 810  60 130 190700  90 700 300 GI Comparative Example 46 R 860 600 560 28800 50.0 810330 250 500 680 110 CR Comparative Example 47 S 900 550 510 36000 46.2850 150 190 190 690 160 720 600 520 CA Example 48 T 870 550 570 1440052.9 850 140 150 170 700  90 CR Example 49 U 905 560 47.1 840 300 170420 710 210 700 180 510 GA Example 50 V 890 610 530 28800 55.6 830 180140 260 690  90 680 300 520 GA Example 51 W 910 540 530 18000 56.3 820180 300 270 680 100 725 200 GI Example 52 X 870 520 520 23400 58.8 800120 160 160 690 230 700 250 GI Example 53 Y 880 500 520 23400 64.7 850120 140 140 710 140 700 270 GI Example 54 Z 900 500 570  9000 62.5 850150 320 200 700 120 690 250 500 GA Example 55 AA 910 580 510 28800 56.3860 160 200 210 710 200 740 300 510 GA Example 56 AB 855 580 53.8 880140 150 180 710 230 720 340 520 GA Example 57 AC 900 560 520 32400 56.3900  90 100 130 680  70 685 180 GI Example 58 AD 900 550 540 10800 56.3850  80 100 125 690  90 680 170 530 GA Example 59 AE 850 550 540 1080056.3 800 120 100 150 680 150 730 160 CR Example 60 AF 880 520 510 1440064.7 840 150 170 195 690 120 690 150 480 GA Example 61 AG 840 500 46.7850 180 100 200 680 160 CR Example 62 AH 860 490 560 23400 50.0 840 240170 300 710 200 680 140 540 GA Example 63 AI 880 500 580 10800 57.1 820300 120 180 690 130 680 150 510 GA Example underlined portion: out ofthe range of the present invention CR: cold rolled steel sheet (withouta coating layer). GI: galvanized steel sheet (without an alloyingtreatment on a galvanizing layer). GA: galvannealed steel sheet

TABLE 4 (Sum of Area Fractions (Area fraction of (Average of RA and MHaving a RA Adjacent to Average Average Mn Circle-equivalent Grain Threeor more F Mn Mn Content Thick- size of 3 μm or more)/ Grains)/ ContentContent in RA)/ Steel ness (Sum of Area Fractions (Area raction in RA inF (Average Mn No. Grade (mm) of All Ra and M) of All RA) (mass %) (mass%) Content in F)  1 A 1.4 0.151 0.427 5.85 1.21 4.83  2 A 1.6 0.1120.504 6.19 1.93 3.21  3 A 1.4 0.127 0.382 5.65 1.88 3.01  4 A 1.2 0.2600.488 5.88 1.94 3.03  5 A 1.2 0.131 0.354 5.86 1.96 2.99  6 A 1.4 0.0710.546 4.67 2.72 1.72  7 A 1.2 0.294 0.521 4.55 3.00 1.52  8 A 1.0 0.1950.576 4.82 2.96 1.63  9 A 1.4 0.291 0.440 4.94 3.01 1.64 10 A 1.4 0.1920.524 5.00 3.03 1.65 11 A 1.2 0.201 0.783 4.91 2.08 2.36 12 A 1.4 0.1370.771 4.32 2.01 2.15 13 A 1.2 0.307 0.702 4.24 1.41 3.01 14 A 1.4 0.1860.700 4.15 2.21 1.88 15 A 1.4 0.371 0.486 5.32 2.25 2.36 16 B 1.4 0.1530.527 5.23 2.08 2.51 17 C 1.6 0.292 0.424 5.65 1.48 3.81 18 A 1.8 0.1330.646 4.94 2.68 1.84 19 A 1.0 0.358 0.531 3.95 2.56 1.54 20 A 1.4 0.3990.574 4.56 2.52 1.81 21 A 1.6 0.327 0.417 4.01 3.14 1.28 22 A 1.6 0.3280.491 3.56 2.87 1.34 23 A 1.2 0.064 0.442 3.99 3.20 1.25 24 A 1.4 0.1340.527 3.86 3.20 1.21 25 A 1.4 0.398 0.414 3.74 3.10 1.21 26 A 1.2 0.3480.450 3.89 2.84 1.37 27 A 1.4 0.170 0.475 3.90 2.80 1.39 28 A 1.2 0.2150.470 3.99 2.15 1.86 29 D 1.4 0.252 0.571 4.67 2.97 2.26 30 E 1.4 0.2440.550 6.37 2.80 2.27 31 F 1.2 0.297 0.440 6.40 2.78 2.30 32 F 1.2 0.2920.485 6.38 2.85 2.24 33 G 1.4 0.291 0.494 5.11 2.32 2.20 34 H 1.4 0.1130.478 9.99 3.01 3.32 35 I 1.6 0.142 0.519 5.60 2.71 2.07 36 J 1.8 0.0950.443 4.95 1.96 2.52 37 K 1.4 0.248 0.532 10.09 3.45 2.92 38 L 1.2 0.2440.450 5.40 2.62 2.06 39 L 1.2 0.296 0.517 5.19 2.46 2.11 40 M 1.4 0.2450.451 6.96 2.22 3.13 41 N 1.2 0.385 0.462 5.30 2.19 2.42 42 O 1.4 0.1300.562 5.44 2.61 2.08 43 P 1.2 0.376 0.500 5.22 2.62 1.99 44 P 1.2 0.2995.17 5.35 2.46 2.18 45 Q 1.2 0.252 0.431 14.21 3.03 4.69 46 R 1.4 0.1000.443 5.12 2.85 1.80 47 S 1.4 0.175 0.508 5.14 3.00 1.71 48 T 1.6 0.2650.468 5.51 2.63 2.10 49 U 18 0.360 0.573 5.55 3.02 1.84 50 V 1.6 0.2560.640 5.66 2.83 2.15 51 W 1.4 0.244 0.444 4.55 2.84 1.60 52 X 1.4 0.1330.515 5.40 2.30 2.35 53 Y 1.2 0.291 0.500 5.93 2.28 2.60 54 Z 1.2 0.1110.570 5.92 2.72 2.18 55 AA 1.4 0.100 0.459 5.61 2.50 2.24 56 AB 1.20.365 0.579 5.87 2.61 2.25 57 AC 1.4 0.250 0.494 5.92 2.92 2.03 58 AD1.4 0.289 0.472 5.58 2.62 2.13 59 AW 1.4 0.285 0.443 5.58 2.70 2.07 60AF 1.2 0.073 0.504 5.26 2.83 1.86 61 AG 1.6 0.354 0.559 5.46 2.56 2.1362 AH 1.4 0.099 0.579 5.74 2.83 2.03 63 AI 1.2 0.289 0.537 5.69 2.891.97 underlined portion: out of the range of the present incention F:fernite, M: as-quenched martensile, RA: retained austenite

TABLE 5 Area Area (Area Fraction of Massive Area Area Area Area Aver-Aver- Frac- Fraction RA)/((Area Fraction Remain- Frac- Frac- Frac- Frac-age age tion of of Lath- of Massice RA) + ing tion of tion of tion oftion of size of size of Massive structured (AreaFraction of Lath- Micro-No. F (%) M (%) TM (%) RA (%) F (um) RA (u) RA (%) RA (%) structuredRA)) structure  1 55.5 19.7 0.4 20.4 5.6 1.9 9.4 11.0 0.46 B, P, θ  253.4 21.1 1.9 19.1 5.0 1.0 9.3 9.8 0.49 B, P, θ  3 54.4 22.0 0.7 22.35.1 1.6 8.5 13.8 0.38 B, P, θ  4 52.6 19.3 2.9 23.2 5.2 2.3 10.1 13.10.44 B, P, θ  5 53.7 18.0 2.9 23.4 5.7 1.6 9.3 14.1 0.40 B, P, θ  6 45.134.6 1.7 14.8 5.3 1.0 7.5 7.3 0.51 B, P, θ  7 45.6 34.2 2.3 16.4 5.5 2.47.7 8.7 0.47 B, P, θ  8 44.4 34.9 2.6 15.2 4.9 2.7 7.4 7.8 0.49 B, P, θ 9 46.0 34.8 1.8 15.3 5.7 0.7 7.0 8.3 0.46 B, P, θ 10 45.9 34.1 0.6 15.95.1 2.6 8.8 7.1 0.55 B, P, θ 11 48.2 25.0 2.5 20.9 5.2 2.5 18.1 2.8 0.87B, P, θ 12 54.1 21.7 0.6 19.8 5.1 1.4 15.5 4.3 0.78 B, P, θ 13 54.9 22.90.8 21.2 5.2 2.1 14.3 6.9 0.67 B, P, θ 14 50.9 20.3 1.1 22.2 5.0 2.815.1 7.1 0.68 B, P, θ 15 51.8 18.4 2.2 24.8 5.1 1.7 7.0 17.8 0.28 B, P,θ 16 56.7 17.5 1.8 21.0 5.7 0.6 6.6 14.4 0.31 B, P, θ 17 59.6 12.3 0.825.1 5.3 0.9 12.6 12.5 0.50 B, P, θ 18 48.1 34.0 2.6 14.4 7.1 0.7 6.48.0 0.44 B, P, θ 19 57.9 15.4 1.0 21.8 9.5 2.1 10.0 11.8 0.46 B, P, θ 2052.4 21.2 2.8 20.5 5.3 2.2 17.1 3.4 0.83 B, P, θ 21 64.3 3.2 1.7 6.6 4.92.5 5.4 1.2 0.82 B, P, θ 22 78.5 10.2 0.2 5.5 6.0 0.9 4.4 1.1 0.80 B, P,θ 23 72.7 4.3 2.1 6.0 5.6 1.8 5.5 0.5 0.92 B, P, θ 24 45.5 38.9 0.1 5.25.3 1.0 5.1 0.1 0.98 B, P, θ 25 78.8 3.2 1.0 6.6 5.5 1.7 4.0 2.6 0.61 B,P, θ 26 46.7 36.4 2.7 13.2 5.0 1.8 6.8 6.4 0.52 B, P, θ 27 75.5 3.5 2.77.1 5.2 2.8 7.1 0.0 1.00 B, P, θ 28 50.0 35.6 1.2 9.3 6.0 0.6 8.1 1.20.87 B, P, θ 29 50.2 18.7 1.1 25.8 4.9 2.3 4.9 20.9 0.19 B, P, θ 30 49.422.1 2.2 18.1 5.1 1.7 9.0 9.1 0.50 B, P, θ 31 47.8 20.3 0.9 24.9 5.4 0.87.2 17.7 0.29 B, P, θ 32 51.0 21.1 2.6 19.1 5.2 2.7 9.4 9.7 0.49 B, P, θ33 49.7 19.4 1.5 25.0 4.8 1.9 12.3 12.7 0.49 B, P, θ 34 55.4 20.4 0.619.6 5.3 1.8 9.8 9.8 0.50 B, P, θ 35 50.3 18.8 1.2 25.9 5.0 1.9 11.114.8 0.43 B, P, θ 36 43.1 34.4 2.1 15.1 5.5 1.6 7.5 7.6 0.50 B, P, θ 3751.8 14.5 1.8 27.1 5.6 0.5 10.4 16.7 0.38 B, P, θ 38 53.3 14.5 0.5 25.95.7 0.8 12.6 13.3 0.49 B, P, θ 39 50.0 15.2 2.1 26.4 6.0 2.1 13.1 13.30.50 B, P, θ 40 52.4 16.6 0.5 26.0 5.4 0.7 12.1 13.9 0.47 B, P, θ 4156.9 18.9 2.7 20.6 5.8 1.5 9.9 10.7 0.48 B, P, θ 42 52.6 19.9 1.8 20.05.3 1.8 8.3 11.7 0.42 B, P, θ 43 57.4 15.8 0.6 24.4 5.7 0.8 12.6 11.80.52 B, P, θ 44 50.2 22.1 2.2 23.4 6.0 2.1 13.1 10.3 0.56 B, P, θ 4549.3 13.3 2.3 29.3 5.3 1.8 14.5 14.8 0.49 B, P, θ 46 50.2 36.3 0.1 12.15.5 0.8 12.0 0.1 0.99 B, P, θ 47 54.3 17.5 2.8 22.9 5.2 2.3 9.2 13.70.40 B, P, θ 48 53.1 17.6 2.5 22.8 5.7 1.4 9.3 13.5 0.41 B, P, θ 49 52.721.6 2.8 21.2 5.5 1.2 6.3 14.9 0.30 B, P, θ 50 50.1 21.8 1.6 23.6 5.70.8 7.4 16.2 0.31 B, P, θ 51 48.1 34.3 1.3 15.8 5.0 2.7 8.6 7.2 0.54 B,P, θ 52 50.2 20.0 1.8 20.4 5.5 1.5 8.8 11.6 0.43 B, P, θ 53 50.4 20.42.6 20.1 5.3 1.5 9.9 10.2 0.49 B, P, θ 54 51.0 20.1 2.8 20.8 5.7 0.5 8.212.6 0.39 B, P, θ 55 53.4 19.7 1.3 20.2 5.0 0.9 9.0 11.2 0.45 B, P, θ 5652.0 20.1 0.7 21.0 5.3 1.1 9.7 11.3 0.46 B, P, θ 57 51.5 20.4 0.4 21.05.5 1.5 10.1 10.9 0.48 B, P, θ 58 50.9 20.0 2.2 22.6 5.4 1.8 9.6 13.00.42 B, P, θ 59 51.0 20.2 0.5 23.2 6.8 1.4 11.1 12.1 0.48 B, P, θ 6053.7 18.7 2.4 20.4 6.0 1.2 9.2 11.2 0.45 B, P, θ 61 52.2 22.2 0.1 21.35.5 2.4 10.0 11.3 0.47 B, P, θ 62 50.3 19.5 1.7 20.4 5.1 1.3 9.9 10.50.49 B, P, θ 63 51.2 23.7 1.9 20.1 5.0 2.3 9.4 10.7 0.47 B, P, θunderlined portion: out of the range of the present invention F:fernite, M: as-quenched martensile, TM: tempered martensile. RA:retained austenite, B: bainite, P: pearlite, θ: carbide (such ascementile)

TS EL TS × EL λ (Blanking) λ (Reaming) R No. (MPa) (%) (MPa %) (%) (%)(mm) R/I Phosphatability Coatablity Note  1 993 22.4 22243 23 53 3.4 2.45 Example  2 1016 24.8 25246 21 45 3.6 2.3 ◯ Example  3 1006 25.8 2600622 55 3.2 2.3 ◯ Example  4 1020 24.5 24990 18 49 2.4 2.0 5 ◯ Eample  51025 27.0 27675 26 56 2.6 2.2 ◯ Example  6 1161 14.0 16534 15 35 3.2 2.35 Example  7 1201 13.7 16454 18 33 2.6 2.2 ◯ Example  8 1230 14.8 1820419 40 2.4 2.4 ◯ Example  9 1197 13.6 16279 22 43 3.4 2.4 ◯ Example 101198 15.0 17970 21 41 3.5 2.5 ◯ Example 11 1050 22.0 23100 14 45 3.8 3.2◯ Comparative Example 12 982 23.9 23470 14 46 3.8 2.7 ◯ ComparativeExample 13 1091 21.1 23020  5 47 4.0 3.3 ◯ Comparative Example 14 105222.3 23460 13 40 4.2 3.0 5 Comparative Example 15 1030 26.5 27295 25 512.8 2.0 ◯ Example 16 1015 25.5 25883 19 60 2.8 2.0 ◯ Example 17 100124.9 24925 20 46 3.5 2.2 ◯ Example 18 1195 10.8 12906 16 42 3.4 1.9 ◯Comparative Example 19 944 29.6 27942 27 58 1.0 1.0 ◯ ComparativeExample 20 1000 23.6 23800 15 42 4.5 2.5 ◯ Example 21 903 25.4 22936 2943 2.0 1.3 ◯ Comparative Example 22 1020 18.7 19074 35 60 3.4 2.1 ◯Comparative Example 23 978 19.6 19169 15 61 2.6 2.2 5 ComparativeExample 24 1209 11.3 13662 10 51 3.4 2.4 ◯ Comparative Example 25 86018.6 15996 33 48 1.4 1.0 ◯ Comparative Example 26 1121 13.9 15582 13 462.8 2.3 ◯ Comparative Example 27 802 18.4 14757 40 59 0.8 0.6 ◯Comparative Example 28 1090 15.1 16459 10 52 3.5 2.5 ◯ ComparativeExample 29 1071 26.4 28274 24 62 2.8 2.0 4 Example 30 980 21.0 20582 2041 3.4 2.4 ◯ Example 31 1049 23.2 24337 21 42 2.5 2.1 4 Example 32 100024.0 24000 20 40 2.4 2.0 Δ Example 33 1053 27.8 29273 26 47 3.2 2.3 ◯Example 34 996 21.5 21414 20 55 3.2 2.3 ◯ Example 35 1023 23.2 23734 2047 3.4 2.1 4 Example 36 1185 15.2 18012 22 36 3.8 2.1 ◯ Example 37 99329.0 28797 27 53 2.5 1.8 ◯ Example 38 987 22.2 21911 21 43 3.0 2.5 ◯Example 39 1001 22.4 22422 20 45 2.8 2.3 4 Example 40 1022 26.0 26572 1846 2.6 1.9 ◯ Example 41 1030 23.1 23793 23 45 2.6 2.2 ◯ Example 42 99923.0 22977 20 46 2.6 2.0 4 Example 43 987 21.6 21319 21 52 2.8 2.3 xComparative Example 44 1084 19.9 21572 18 50 2.4 2.0 2 ComparativeExample 45 984 30.9 30406 21 43 2.8 2.3 x Comparative Example 46 109221.0 22932 10 48 3.2 2.3 4 Comparative Example 47 993 21.4 21250 24 542.8 2.0 ◯ Example 48 994 24.9 24751 23 60 4.0 2.5 4 Example 49 1034 24.325126 22 58 4.2 2.3 ◯ Example 50 1099 22.3 24508 20 44 3.0 1.9 ◯ Example51 1214 15.0 18210 16 38 2.6 1.9 ◯ Example 52 999 26.9 26873 22 52 3.02.1 ◯ Example 53 984 23.1 22730 23 53 2.6 2.2 ◯ Example 54 1020 21.321726 20 51 2.4 2.0 ◯ Example 55 995 24.0 23880 22 53 3.0 2.1 ◯ Example56 983 21.1 20741 24 45 2.4 2.0 ◯ Example 57 984 22.0 21646 23 49 3.02.1 ◯ Example 58 1037 20.2 20947 21 42 3.2 2.3 ◯ Example 59 1000 22.822800 23 53 3.2 2.3 4 Example 60 1012 22.8 23074 22 51 2.5 2.1 ◯ Example61 1080 17.8 19224 19 42 3.8 2.4 4 Example 62 982 25.1 24648 25 50 2.82.0 ◯ Example 63 1062 23.0 24426 21 50 2.5 2.1 ◯ Example underlinedportion: out of the range of the present invention

A tensile test was performed on a JIS No. 5 test specimen which had beentaken as a sample so that the tensile direction was a directionperpendicular to the rolling direction of the steel sheet in accordancewith JIS Z 2241 (2011) to determine TS (tensile strength) and EL (totalelongation).

In the present invention, the following cases in mechanical propertiesare judged as good.

EL: 20% or more in the case of TS: 980 MPa or more and less than 1080MPa

EL: 16% or more in the case of TS: 1080 MPa or more and less than 1180MPa

EL: 12% or more in the case of TS: 1180 MPa or more and less than 1270MPa

Here, the thickness was 1.0 mm to 1.8 mm.

A hole expanding test was performed in accordance with JIS Z 2256(2010). Each of the obtained steel sheet was cut into a piece having asize of 100 mm×100 mm, a hole having a diameter of 10 mm was made in thecut piece by using a blanking method with a clearance of 12%±1% or areaming method, and the hole was expanded by pushing a conical punchhaving a point angle of 60° into the hole while the cut piece was heldwith a blank holding force of 9 tons on a die having an inner diameterof 75 mm to determine the diameter of the expanded hole for a crackgeneration limit, the limiting hole expanding ratio λ (%) was calculatedby using the equation below, and hole expansion formability wasevaluated on the basis of the limiting hole expanding ratio.

Here, the term “reaming” denotes a process in which a drilled hole isenlarged by using the cutting edges of a reamer so that the hole has apredetermined diameter, and the cut surface is then finished by usingthe margin of the reamer so that the cut surface is smoothened.

limiting hole expanding ratio ((λ)={(D _(f) −D ₀)/D ₀}×100

Here, D_(f) denotes the diameter (mm) of the hole when a crack isgenerated, and D₀ denotes the initial diameter (mm) of the hole. Here,in the present invention, the following cases are judged as gooddepending on TS.

(blanking) λ: 15% or more or (reaming) λ: 40% or more in the case of TS:980 MPa or more and less than 1080 MPa

(blanking) λ: 12% or more or (reaming) λ: 35% or more in the case of TS:1080 MPa or more and less than 1180 MPa

(blanking) λ: 10% or more or (reaming) λ: 30% or more in the case of TS:1180 MPa or more and less than 1270 MPa

A bending test was performed on a bending test specimen having a widthof 30 mm and a length of 100 mm which had been taken from each of theannealed steel sheets so that the rolling direction was the bendingdirection in accordance with the V-block method prescribed in JIS Z 2248(1996). The test was performed with a pushing speed of 100 mm/s threetimes each for respective one of the bending radii, the outer side ofthe bending position was observed by using a stereoscopic microscope tojudge whether or not a crack was generated, and the limiting bendingradius R (mm) was defined as the minimum bending radius with which nocrack was generated. Here, in the present invention, a case where(limiting bending radius R)/t (t: steel sheet thickness (mm)) was 2.5 orless in a 90° V-bend was judged as a case of a steel sheet having goodbendability.

Phosphatability was evaluated by forming a chemical conversion coatingfilm by performing a chemical conversion treatment on each of theobtained cold rolled steel sheet by using the method described below inwhich a chemical conversion treatment solution produced by NihonParkerizing Co., Ltd. (PALBOND L-3080 (registered trademark)) was used.

The obtained cold rolled steel sheet was first degreased by using adegreasing solution FINE CLEANER (registered trademark) produced byNihon Parkerizing Co., Ltd., washed with water, and then subjected tosurface conditioning for 30 s by using a surface conditioning solutionPREPALENE-Z (registered trademark) produced by Nihon Parkerizing Co.,Ltd. The cold rolled steel sheet which had been subjected to surfaceconditioning was dipped in a chemical conversion treatment solution(PALBOND L-3080 (registered trademark)) having a temperature of 43° C.for 120 s, washed with water, and dried with hot air. In such a way achemical conversion treatment was performed on the cold rolled steelsheet.

The surface of the cold rolled steel sheet which had been subjected to achemical conversion treatment was observed by using a SEM (scanningelectron microscope) at a magnification of 500 times in randomlyselected 5 fields of view. The area fraction (i) of a region in which achemical conversion film was not formed (so called lack of hiding) wasdetermined by performing image analysis, and evaluation was performed onthe basis the obtained area fraction in accordance with the followingevaluation criteria.

grade 5: 5% or less

grade 4: more than 5%, and 10% or less

grade 3: more than 10% and 25% or less

grade 2: more than 25% and 40% or less

grade 1: more than 40%

A case of grade 4 or grade 5 is regarded as a case of goodphosphatability. In particular, grade 5 is preferable.

Coatability was evaluated by performing visual test. A case where anappropriate surface quality is achieved without any poor appearance suchas a coating defect, a variation in alloying degree, and other defectscausing deterioration in surface quality was judged as “O”, a case of anexcellent appearance without a variation in color tone or the like wasjudged as “0”, a case where a partial minor defect was found was judgedas “A”, and a case where many surface defects were found was judged as“x”.

All of the high strength steel sheets of the examples of the presentinvention had a TS of 980 MPa or more and excellent formability. On theother hand, the comparative examples were poor in terms of at least oneof TS, EL, λ, bendability, phosphatability, and coatability.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a highstrength steel sheet having a TS (tensile strength) of 930 MPa or moreand excellent formability. By using the high strength steel sheetaccording to the present invention for, for example, automobilestructural members, it is possible to improve fuel efficiency due to adecrease in the weight of an automobile body, which has a significantutility value in the industry.

1. A high strength steel sheet comprising a chemical compositioncontaining, by mass %, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn:2.50% to 8.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005%to 0.0100%, Al: 0.001% to 2.000%, and a balance being Fe and incidentalimpurities, and a steel microstructure including, in terms of areafraction, 35% or more and 80% or less of ferrite, 5% or more and 35% orless of as-quenched martensite, 0.1% or more and less than 3.0% oftempered martensite, and 8% or more of retained austenite, in which anaverage grain size of the ferrite is 6 μm or less, in which an averagegrain size of the retained austenite is 3 μm or less, in which a valuecalculated by dividing an average Mn content (mass %) in the retainedaustenite by an average Mn content (mass %) in the ferrite is 1.5 ormore, in which a value calculated by dividing a sum of an area fractionof as-quenched martensite having a circle-equivalent grain size of 3 μmor more and an area fraction of retained austenite having acircle-equivalent grain size of 3 μm or more by a sum of an areafraction of all the as-quenched martensite and an area fraction of allthe retained austenite is less than 0.4, and in which a value calculatedby dividing an area fraction of retained austenite grains adjacent tothree or more ferrite grains having different crystal orientations bythe area fraction of all the retained austenite is less than 0.6.
 2. Thehigh strength steel sheet according to claim 1, wherein the chemicalcomposition further contains, by mass %, at least one selected from Ti:0.005% to 0.200%, Nb: 0.005% to 0.200%, V: 0.005% to 0.500%, W: 0.005%to 0.500%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%, Cr: 0.005% to1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002% to0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.100%, Ca: 0.0005% to0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM:0.0005% to 0.0050%.
 3. The high strength steel sheet according to claim1 or 2, wherein the steel sheet has a galvanizing layer on a surface ofthe steel sheet.
 4. The high strength steel sheet according to claim 3,wherein the galvanizing layer is a galvannealing layer.
 5. A method formanufacturing a high strength steel sheet, the method comprising heatinga steel slab having the chemical composition according to claim 1 or 2,performing hot rolling on the heated slab with a finish rolling deliverytemperature of 750° C. or higher and 1000° C. or lower, coiling the hotrolled steel sheet at a temperature of 300° C. or higher and 750° C. orlower, performing cold rolling on the hot rolled steel sheet,subsequently holding the cold rolled steel sheet in a temperature rangeequal to or higher than an Ac₃ transformation temperature for 20 s to1800 s, cooling the steel sheet to a cooling stop temperature of 50° C.or higher and 350° C. or lower, holding the cooled steel sheet at thecooling stop temperature for 2 s to 600 s, cooling the steel sheet,subsequently holding the cooled steel sheet in a temperature range equalto or higher than an Ac₁ transformation temperature and equal to orlower than (Ac₁ transformation temperature+150° C.) for 20 s to 1800 s,and cooling the steel sheet.
 6. A method for manufacturing a highstrength steel sheet, the method comprising heating a steel slab havingthe chemical composition according to claim 1 or 2, performing hotrolling on the heated slab with a finish rolling delivery temperature of750° C. or higher and 1000° C. or lower, coiling the hot rolled steelsheet at a temperature of 300° C. or higher and 750° C. or lower,performing cold rolling on the hot rolled steel sheet, subsequentlyholding the cold rolled steel sheet in a temperature range equal to orhigher than an Ac₃ transformation temperature for 20 s to 1800 s,cooling the steel sheet to a cooling stop temperature of 50° C. orhigher and 350° C. or lower, holding the cooled steel sheet at thecooling stop temperature for 2 s to 600 s, subsequently cooling thesteel sheet, holding the cooled steel sheet in a temperature range equalto or higher than an Ac₁ transformation temperature and equal to orlower than (Ac₁ transformation temperature+150° C.) for 20 s to 1800 s,cooling the steel sheet, again holding the cooled steel sheet in thetemperature range equal to or higher than the Ac₁ transformationtemperature and equal to or lower than (Ac₁ transformationtemperature+150° C.) for 20 s to 1800 s, and cooling the steel sheet. 7.The method for manufacturing a high strength steel sheet according toclaim 5 or 6, the method further comprising, after coiling has beenperformed, holding the steel sheet in a temperature range equal to orlower than the Ac₁ transformation temperature for more than 1800 s. 8.The method for manufacturing a high strength steel sheet according toany one of claims 5 to 7, the method further comprising performing agalvanizing treatment.
 9. The method for manufacturing a high strengthsteel sheet according to claim 8, the method further comprising, afterthe galvanizing treatment has been performed, performing an alloyingtreatment at a temperature of 450° C. to 600° C.